I want to turn a simple idea—earning money directly from my phone—into a fully-featured buy-and-sell application available on both Android and iOS. The core of the product is a smooth marketplace where users can list items quickly, pay safely, and communicate easily, so the build must include: • Secure pembayaran online that supports the most common Indonesian gateways as well as cards and e-wallets. • A reliable sistem ulasan pengguna to build trust between buyers and sellers. • Obrolan langsung dengan penjual, preferably real-time, so negotiations and after-sales questions feel natural. Clean, modern UI/UX is important, but performance and data security come first. I am open to native builds or a cross-platform stack such as Flutter or React Native, as long as the final apps run flawlessly on both operating systems and are ready for submission to Google Play and the App Store. An admin dashboard for user management, listings moderation, and transaction tracking will also be required. Please outline your proposed tech stack, approximate timeline, and any similar marketplace work you have shipped so I can gauge fit and move forward quickly.